1. Field of the Invention
This invention relates to a coil spring manufacturing apparatus for manufacturing coil springs by winding a wire on a mandrel.
2. Description of the Prior Art
As shown in FIGS. 4(A), (B), a conventional apparatus for manufacturing coil springs includes a pair of feed rollers 30a, 30b arranged to feed a wire 33 through a guide 31 in such a manner that the distal end of the wire 33 is engaged by a projection 32a on a winding tool 32 comprising a mandrel. The winding tool 32 is rotated while being moved along its axis, whereby the wire is wound upon the tool and thus formed into a coil spring. A cutter 34 is actuated at the completion of coil spring formation to sever the wire 33.
In the above-described conventional apparatus, the feed rollers 30a, 30b and the winding tool 32 are driven rotatively by a single motor acting through a mechanism consisting of such components as gears and clutches. Owing to such a structural arrangement, it is extremely difficult to establish good agreement between the amount of wire fed by the feed rollers 30a, 30b and the amount of wire wound up by the winding tool 32. It has been attempted to solve this problem by disengaging the feed rollers 30a, 30b from a drive shaft when the wire 33 is taken up by the winding tool 32, thus subjecting the wire 33 to a prescribed tension derived from the inertia possessed by the feed rollers proper. However, such an expedient is attended by a number of difficulties. For example, depending upon such factors as the winding rate of the winding tool 32, the material constituting the wire 33 and the rigidity of the wire, a difference in the amount of wire bulge develops, as shown in FIG. 4(A), even though winding of the wire is intended to be wound around the winding tool 32, as shown in FIG. 4(C). As a result, even though the wire 33 is wound upon the tool 32, which will take place providing there is no slip, an error in the winding angle is produced. This makes it impossible to manufacture coil springs of identical quality. In addition, though the wire 32 will be taken up by the mandrel or winding tool 32 in a reliable manner if the feed rollers 30a, 30b tension the wire 33 weakly, too much tension does not enable positive take up, thus resulting in a shortened leg 33a, as shown in FIG. 4(B). Coil springs manufactured by such an arrangement therefore do not have legs 33a of identical length. Furthermore, since the wire 33 is drawn out owing to the tension between the wire and the projection 32a of the winding tool 32, the projection 32a is subjected to a load and presents a degree of friction that can have a detrimental effect upon product stability. Also, a slight amount of play which develops between the wire 33 and the projection 32a diminishes the length of the coil spring leg 33a correspondingly.